Microstructural Origin of Irradiation Hardening and Embrittlement in Irradiated Second Generation FeCrAl Alloys

FeCrAl alloys are promising candidate materials for the accident tolerant fuel (ATF) cladding application in Light Water Reactors (LWRs) due to their exceptional corrosion resistance in elevated temperature steam environments. Previous results suggest that the fracture toughness of FeCrAl alloys may be a factor in its deployment for nuclear power applications and FeCrAl undergoes severe embrittlement at low temperatures near end-of-life conditions. However, there is little information on the microstructural origin of these observations. Previous studies suggested intermetallic precipitates from the minor alloying elements, such as Mo and Si, were responsible for crack formation under tensile stress in as-fabricated FeCrAl but may dissolve or undergo amorphization under irradiation. Additionally, dislocation loop formation under irradiation is expected to contribute significantly to embrittlement. Thus, characterizing the dislocation loop microstructure and the radiation response of secondary phases in FeCrAl is critical to understanding their irradiated properties. We propose to investigate the relationship between the irradiated microstructure and irradiation hardening and embrittlement of two irradiated FeCrAl alloys near end-of-life conditions using detailed post irradiation electron microscopy and nanoindentation. We will investigate the hypothesis that embrittlement is dominated by small dislocations and amorphized particles at low irradiation temperatures with both pre-existing dislocations and precipitates dissolving at high irradiation temperatures to soften the alloy. Candidate Generation II FeCrAl alloys, C06M and C36M, representing the bounding compositions of FeCrAl originally pursued for ATF applications, were irradiated in the High Flux Isotope Reactor (HFIR) at nominal temperatures of 200, 330 and 550 °C to about 18 dpa. The proposing team seeks to use, through the Nuclear Science User Facilities, the Low Activation Materials Development and Analysis (LAMDA) facility at ORNL for sample preparation, nanoindentation and detailed post irradiation electron microscopy. Nanoindentation will produce hardness data to compare with previous Vickers microhardness and inform locations for lamella extraction. TEM lamella will be produced for two alloys irradiated at 3 different temperatures for subsequent scanning transmission electron microscopy (STEM). On-zone STEM imaging and energy dispersive x-ray spectroscopy (EDS) will be used to characterize dislocations and secondary phases. There are 6 samples and thus, in total, the proposed experiments will require an estimated 24 hours for nanoidentation, 36 hours for lamella preparation and 60 hours for post-irradiation examination. Quantitative size distributions for dislocation line density, dislocation loops and precipitating phases will be obtained and correlated to the radiation embrittlement through the Vickers microhardness and a dispersed barrier hardening model. Completion of the proposed study will provide several outcomes: produce quantitative analysis of the irradiated microstructure of candidate ATF alloys, compare microhardness and nanohardness to link the techniques across length scales, determine contributions of different microstructural features to the irradiation induced embrittlement, deepen the understanding between the irradiated microstructure, hardness, and fracture toughness, and thus from these, fill a critical knowledge gap in microstructure-property relationships for FeCrAl alloys. This knowledge is critical to understanding how the minor element composition can be tailored to reduce embrittlement in FeCrAl alloys for LWR applications.